5,329 research outputs found
Exact Chern-Simons / Topological String duality
We invoke universal Chern-Simons theory to analytically calculate the exact
free energy of the refined topological string on the resolved conifold. In the
unrefined limit we reproduce non-perturbative corrections for the resolved
conifold found elsewhere in the literature, thereby providing strong evidence
that the Chern-Simons / topological string duality is exact, and in particular
holds at arbitrary N as well. In the refined case, the non-perturbative
corrections we find are novel and appear to be non-trivial. We show that
non-perturbatively special treatment is needed for rational valued deformation
parameter. Above results are also extend to refined Chern-Simons with
orthogonal groups.Comment: 32 page
Cosmological constraints for a two brane-world system with single equation of state
We present the study of two 3-brane system embedded in a 5-dimensional
space-time in which the fifth dimension is compactified on a
orbifold. Assuming isotropic, homogeneous, and static branes, it can be shown
that the dynamics of one brane is dominated by the other one when the metric
coefficients have a particular form. We study the resulting cosmologies when
one brane is dominated by a given single-fluid component.Comment: This work was presented at the VIII Taller of the DGFM, Tuxtla
Gutierrez, Chiapas, Mexico noviembre 22-26, 201
An equation of state for purely kinetic k-essence inspired by cosmic topological defects
We investigate the physical properties of a purely kinetic k-essence model
with an equation of state motivated in superconducting membranes. We compute
the equation of state parameter and discuss its physical evolution via a
nonlinear equation of state. Using the adiabatic speed of sound and energy
density, we restrict the range of parameters of the model in order to have an
acceptable physical behavior. Furthermore, we analyze the evolution of the
luminosity distance with redshift by comparing (normalizing) it
with the CDM model. Since the equation of state parameter is
-dependent the evolution of the luminosity distance is also analyzed using
the Alcock-Paczy\'{n}ski test.Comment: 19 pages, 13 figures, typos corrected and references adde
Two-brane system in a vacuum bulk with a single equation of state
We study the cosmology of a two-brane model in a five-dimensional spacetime,
where the extra spatial coordinate is compactifed on an orbifold. Additionally,
we consider the existence on each brane of matter fields that evolve in time.
Solving the Einstein equations in a vacuum bulk, we can show how the matter
fields in both branes are connected and they do not evolve independentlyComment: This work was presented at the IX Taller de la Division de
Gravitacion y Fisica Matematica, Colima 201
Mixed spatially varying -BV regularization of inverse ill-posed problems
Several generalizations of the traditional Tikhonov-Phillips regularization
method have been proposed during the last two decades. Many of these
generalizations are based upon inducing stability throughout the use of
different penalizers which allow the capturing of diverse properties of the
exact solution (e.g. edges, discontinuities, borders, etc.). However, in some
problems in which it is known that the regularity of the exact solution is
heterogeneous and/or anisotropic, it is reasonable to think that a much better
option could be the simultaneous use of two or more penalizers of different
nature. Such is the case, for instance, in some image restoration problems in
which preservation of edges, borders or discontinuities is an important matter.
In this work we present some results on the simultaneous use of penalizers of
and of bounded variation (BV) type. For particular cases, existence and
uniqueness results are proved. Open problems are discussed and results to
signal restoration problems are presented.Comment: 18 pages, 12 figure
Advanced nozzle and engine components test facility
A test facility for conducting scaled advanced nozzle and engine component research is described. The CE-22 test facility, located in the Engine Research Building of the NASA Lewis Research Center, contains many systems for the economical testing of advanced scale-model nozzles and engine components. The combustion air and altitude exhaust systems are described. Combustion air can be supplied to a model up to 40 psig for primary air flow, and 40, 125, and 450 psig for secondary air flow. Altitude exhaust can be simulated up to 48,000 ft, or the exhaust can be atmospheric. Descriptions of the multiaxis thrust stand, a color schlieren flow visualization system used for qualitative flow analysis, a labyrinth flow measurement system, a data acquisition system, and auxiliary systems are discussed. Model recommended design information and temperature and pressure instrumentation recommendations are included
ESTIMATING INVESTMENT RIGIDITY WITHIN A THRESHOLD REGRESSION FRAMEWORK: THE CASE OF U.S. HOG PRODUCTION SECTOR
As the U.S. hog production sector becomes ever more specialized, the importance of capital inputs has heightened. Given that it is costly to adjust the capital stock and that the associated adjustment cost function may exhibit cost asymmetries between investment and disinvestment, profit-maximizing producers may find themselves trapped in a situation where it is neither profitable investing nor worthwhile disinvesting. This article addresses two issues related to the employment of quasi-fixed input in the U.S. hog production sector: does an inaction or sluggish regime exist in the demand for quasi-fixed input, and, if so, to what extent has this impeded adjustment in quasi-fixed input stock and, hence, hog output supply toward the long-term equilibrium levels? The conceptual framework is based on the work by Abel and Eberly and allows for the existence of an inaction/sluggish regime, alongside an investment regime and a disinvestment regime. Quarterly data from 1976 through 1999 are used to estimate the three-regime investment demand equation, treating breeding sows as the quasi-fixed input. The threshold estimation procedure recently advanced by Hansen is adopted. To provide a linkage between breeding herd investment and hog output supply, a hog supply equation, specified in part as a function of lagged breeding stock, is estimated by a least squares procedure. The dynamic recursive system of investment demand and hog supply is used to simulate the effects on breeding stock and hog supply of changes in the magnitude of investment rigidities. The econometric results strongly support the three-regime breeding herd investment model. More than 10 percent of the observations fall into the sluggish regime, indicating that this regime has occurred sufficiently often to warrant attention. The estimated rate of adjustment toward the long-run equilibrium breeding stock is 2.7 percent per quarter. The existence of a linkage between lagged breeding stock and hog supply is confirmed. Thus, the results suggest that it is important to account for investment rigidity when estimating breeding herd demand and hog supply. Simulation results indicate that the effects on breeding stock and hog supply of continued specialization in the hog production sector may not be as significant as what the hog production sector has experienced in the past decades. More importantly the simulations suggest that the impact of increasing investment rigidity is rather modest, about 3 percent at most and, thus, no policy intervention appears to be needed. However, the econometric results clearly indicate that estimates will be biased if investment rigidity is not explicitly accounted for when estimating breeding herd demand and hog supply.Livestock Production/Industries,
Nonlinear Criterion for the Stability of Molecular Clouds
Dynamically significant magnetic fields are routinely observed in molecular
clouds, with mass-to-flux ratio lambda = (2 pi sqrt{G}) (Sigma/B) ~ 1 (here
Sigma is the total column density and B is the field strength). It is widely
believed that ``subcritical'' clouds with lambda < 1 cannot collapse, based on
virial arguments by Mestel and Spitzer and a linear stability analysis by
Nakano and Nakamura. Here we confirm, using high resolution numerical models
that begin with a strongly supersonic velocity dispersion, that this criterion
is a fully nonlinear stability condition. All the high-resolution models with
lambda <= 0.95 form ``Spitzer sheets'' but collapse no further. All models with
lambda >= 1.02 collapse to the maximum numerically resolvable density. We also
investigate other factors determining the collapse time for supercritical
models. We show that there is a strong stochastic element in the collapse time:
models that differ only in details of their initial conditions can have
collapse times that vary by as much as a factor of 3. The collapse time cannot
be determined from just the velocity dispersion; it depends also on its
distribution. Finally, we discuss the astrophysical implications of our
results.Comment: 11 pages, 5 figure
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